Generally power amplifiers are characterized by such factors as gain, input noise figure, bandwidth, operating frequency, and distortion characteristics. The distortion characteristics may be expressed as second and third order intercept points, and by AM/AM and AM/PM conversion characteristics. Usually these characteristics are determined during the development and production test of the amplifiers, and are not measured when the amplifier is in operation in a transmitter or other device.
In part, these measurements are not performed in normal operation as most of the measuring techniques involve interruption of the transmissions or require use of an unmodulated signal.